“Percy Jackson: Sea of Monsters”: By gods, this is an unnecessary sequel

And you think you’ve got daddy issues. What if you were the half-blood son of the Greek god Poseidon, ruler of the sea? Every time you needed to have a heart-to-heart with Pops, you had to go talk to the ocean. And, even more humiliatingly, the ocean never talks back.

That’s the predicament that Percy Jackson finds himself in in Rick Riordan’s wildly popular series of young adult novels, now being adapted for the movies. But the second film, “Sea of Monsters,” seems to be a downmarket version of 2010’s “The Lightning Thief.” While the highly similar Harry Potter books and films seemed to deepen and darken their themes with every film, Percy seems to be going shallower and flashier.

Percy and other demigod offspring are safely esconced at Camp Half-Blood, run by Dionysus (a hilarious Stanley Tucci) and centaur Chiron (Anthony Head, taking over for Pierce Brosnan from the first film.) Percy and his friends, including Alexandra Daddario as Annabeth and Brandon T. Jackson as a wisecracking satyr. But when the magical barrier protecting the camp starts to falter, Percy pulls his friends into a quest to find the Golden Fleece to heal it.

Which sounds exciting, but what “Sea of Monsters” really skimps on is 1. Sea and 2. Monsters. Instead, there’s a lot of backstory and exposition as Percy and his team make their way to the sea, lots of stuff about destiny and sins of the fathers that you can’t believe couldn’t have been streamlined. One bright spot is a stop off to see Hermes (a witty Nathan Fillion) who runs sort of a UPS for Greek gods. (When he’s omnipotent, brown can do an awful lot for you.)

The monsters include a cyclops, a mechanical bull and, most niftily of all, a werewolf-like creature with a scorpion’s stinging tail. But Percy spends more time fighting fellow half-bloods, especially the sneering Luke (Jake Abel), who wants to use the Fleece to raise a vengeful god to destroy Mount Olympus. There’s fun to be had here — the tone is much lighter and jokier than the Harry Potter films — but there’s never a sense of stakes, that any of this really amounts to anything more than a CGI distraction.

Which is too bad, because Lerman is a very likable and unassuming hero (he was great in “The Perks of Being a Wallflower”) and is clearly eager to go a little deeper into Percy’s emotions, to wrestle with the implications of having a deadbeat dad who rules the ocean. Tucci is also highly enjoyable, his Dionysus looking like a ’70s porn director, chafing under a curse by Zeus that turns all his beloved wine into water. “The Christians have a guy who can do this trick in reverse,” he bemoans. “Now THAT’S a God!” But they both seem stranded in a franchise that, for all its grand talk of destiny and omnipotency, seems to be trying to get away with doing it on the cheap.

By the way, I saw “Percy” in 2D, and hear that the 3D version is one of the worst 3D conversions since the first “Clash of the Titans.” It’s utterly unnecessary.